Target rotation determination by speckle motion characterization

ABSTRACT

Systems and corresponding methods for use in measuring rotation characteristics (e.g., rotation magnitude and direction) of remote targets are provided. A laser light of a known frequency is incident upon the target and reflected. A portion of the reflected laser light is directed to detector field of view, where it is measured and analyzed. The detector field of view is divided into multiple segments, each capable of independently measuring the intensity of laser light incident thereon as a function of time. The linear rotation of the target may be determined from cross-correlation of the light intensity-time response measured at orthogonal pairs of detector halves arranged from combinations of the detector segments. The angular rotation of the target is further determined from this linear rotation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/450,888, filed on Aug. 4, 2014 and entitled “TARGET ROTATIONDETERMINATION BY SPECKLE MOTION CHARACTERIZATION,” the entirety of whichis hereby incorporated by reference.

BACKGROUND

Long range target identification plays an important role in defense andspace awareness. Any information that may be obtained from a remotetarget can be potentially used to establish specific features of thetarget and/or classify the target. An example of such information is therotation characteristics of the target, including whether the target isrotating, the magnitude of that rotation, and the axis of that rotation.For example, a target in flight may be observed to exhibit rotation ifspinning about an axis and/or experiencing a change in flight direction.This rotation information alone may be sufficient to determine specificfeatures of the target. Alternatively, this rotation information can beused in combination with other, more sophisticated, imaging approaches(e.g., inverse synthetic aperture and tomography) to determine specificfeatures of the target.

While radar has been employed for measurement of target rotationcharacteristics, radar sensing does not perform well with dynamictargets that rotate relatively slowly. For example, a radar requiresabout 10 minutes to resolve the rotation characteristics of a targetexhibiting an angular velocity of 1 mrad/sec. Such a timeframe may beunacceptably long for certain applications of interest (e.g., targetidentification).

Accordingly, there exists a need for improved techniques for measurementof a target's rotation characteristics at longer ranges.

SUMMARY

In an embodiment, a method of remotely measuring rotation of a target isprovided. The method includes measuring, at a detector having a field ofview divided into at least four spatially distinct light sensingsegments, coherent light reflected from a target incident upon thedetector field of view; outputting, by the detector, a plurality ofsignals to one or more processors, each signal representing a Dopplerspread of the reflected light received at a respective detector segment.The method further includes determining, by the one or more processors:a first Doppler spread for reflected light received at a first half ofthe detector, the first Doppler spread including a first Doppler shift;a second Doppler spread for reflected light received at second half ofthe detector corresponding to the first half, the second Doppler spreadincluding a second Doppler shift, where the first and second halves ofthe detector are separated along a first detector axis of division; anda first Doppler shift difference between the first and second Dopplershifts The method further includes determining, by the one or moreprocessors: a third Doppler spread for reflected light received at athird half of the detector, the third Doppler spread including a thirdDoppler shift; and a fourth Doppler spread for reflected light receivedat a fourth half of the detector corresponding to the third half, thefourth Doppler spread including a fourth Doppler shift; and a secondDoppler shift difference between the third and fourth Doppler shifts;where the third and fourth halves of the detector are separated along asecond detector axis of division orthogonal to the first detector axisof division. The method also includes identifying, by the one or moreprocessors, a rotation axis of the target, the rotation axis alignedwith a vector given by the sum of: a first vector aligned with the firstdetector axis of division and having a magnitude given by the firstDoppler shift difference; and a second vector aligned with the seconddetector axis of division and having a magnitude given by the secondDoppler shift difference.

Embodiments of the method may include one or more of the following, inany combination.

In an embodiment of the method, measuring coherent light reflected fromthe target includes heterodyne detection of the coherent light reflectedfrom the target incident upon the detector field of view.

In an embodiment of the method, determining the Doppler spread forreflected light received at each of said first, second, third, andfourth detector halves includes, by the one or more processors,coherently adding the Doppler spreads measured by at least two differentdetector segments forming said first, second, third, or fourth detectorhalf, respectively.

In an embodiment of the method, the first detector half includes a firstand a second detector segment; the second detector half includes a thirdand a fourth detector segment; the third detector half includes thefirst and the third detector segment; and the fourth detector halveincludes the second and the fourth detector segments.

In an embodiment, the method further includes, by the one or moreprocessors: identifying a width, Δf, of the Doppler spread of thereflected light received over the entire field of view of the detector;and calculating the magnitude of the target rotation according to:Δf=2DΩ/λ, where D is the diameter of the target, Ω is the magnitude ofthe target rotation, and λ is the wavelength of the incident light.

In an embodiment of the method, determining the Doppler spread forreflected light received over the entire field of view of the detectorincludes, by the one or more processors, coherently adding the Dopplerspreads measured for each detector segment.

In an embodiment of the method identifying Δf includes, by the one ormore processors: identifying the peak of the Doppler spread in thefrequency domain; and measuring the full width of the Doppler spread athalf the intensity from the peak; and identifying Δf as said measuredfull width of the Doppler spread.

In an embodiment, a system for measuring rotation of a target isprovided. The system includes a detector including a field of viewdivided into at least four spatially distinct light sensing elements.The detector is adapted to: measure coherent light reflected from atarget incident upon the detector field of view; and output a pluralityof signals, each signal representing a Doppler spread of the reflectedlight received at a respective detector segment. The system furtherincludes one or more processors in communication with the detector. Theone or more processors are adapted to: receive the plurality of signals;determine a first Doppler spread for reflected light received at a firsthalf of the detector, the first Doppler spread including a first Dopplershift; determine a second Doppler spread for reflected light received atsecond half of the detector corresponding to the first half, the secondDoppler spread including a second Doppler shift, where the first andsecond detectors are divided by a first detector axis of division;determine a first Doppler shift difference between the first and secondDoppler shifts; determine a third Doppler spread for reflected lightreceived at a third half of the detector, the third Doppler spreadincluding a third Doppler shift; and determine a fourth Doppler spreadfor reflected light received at a fourth half of the detectorcorresponding to the third half, the fourth Doppler spread including afourth Doppler shift, where the third and fourth halves of the detectorare separated along a second detector axis of division orthogonal to thefirst detector axis of division. The one or more processors are furtheradapted to: determine a second Doppler shift difference between thethird and fourth Doppler shifts; and identify, a rotation axis of thetarget aligned with a vector given by the sum of: a first vector alignedwith the first detector axis of division and having a magnitude given bythe first Doppler shift difference; and a second vector aligned with thesecond detector axis of division and having a magnitude given by thesecond Doppler shift difference.

Embodiments of the system may include one or more of the following, inany combination.

In an embodiment, the system further includes a first light sourceadapted to emit a first coherent light beam having a first frequency; aplurality of optical focusing systems adapted to direct the firstcoherent light beam incident upon the target, and direct least a portionof the coherent light reflected from the target at reflected coherentlight upon the detector field of view; and a second light source adaptedto emit a second coherent light beam having a second frequency upon thedetector field of view, the second frequency different than the firstfrequency; where the plurality of signals output by the detector arebased upon interference of the reflected first coherent light beam andthe second coherent light beam.

In an embodiment of the system, the one or more processors are furtheradapted to coherently add the Doppler spreads measured by at least twodifferent detector segments forming each of said first, second, third,or fourth detector half to determine the Doppler spread for reflectedlight received at each of said first, second, third, and fourth detectorhalves, respectively.

In an embodiment of the system, the detector includes four spatiallydistinct light sensing segments arranged in quadrants, where the firsthalf of the detector includes the first and second detector segments,where the second half of the detector includes the third and fourthdetector segments, where the third half of the detector includes thefirst and third detector segments, and where the fourth half of thedetector includes the second and fourth detector segments.

In an embodiment of the system, the one or more processors are furtheradapted to: identify a width, Δf, of the Doppler spread of the reflectedlight received over the entire field of view of the detector; andcalculate the magnitude of the target rotation according to: Δf=2DΩ/λ,where D is the diameter of the target, Ω is the magnitude of the targetrotation, and λ is the wavelength of the incident light.

In an embodiment of the system, the one or more processors are furtheradapted to determine the Doppler spread for reflected light receivedover the entire field of view of the detector by coherently adding theDoppler spreads measured for each detector segment.

In an embodiment of the system, the one or more processors are furtheradapted to: identify the peak of the Doppler spread in the frequencydomain; and measure the full width of the Doppler spread at half theintensity from the peak; and identify Δf as said measured full width ofthe Doppler spread.

In an embodiment, a non-transitory computer-readable medium is provided.The computer-readable medium includes computer-readable program codesembedded thereon having instructions that, when executed by the one ormore processors, cause the one or more processors to: receive aplurality of signals output from respective segments of a detectorincluding at least four spatially distinct light sensing segments, eachsignal representing a Doppler spread of coherent light reflected from atarget and received at the respective detector segment; determine afirst Doppler spread for reflected light received at a first half of thedetector, the first Doppler spread including a first Doppler shift;determine a second Doppler spread for reflected light received at secondhalf of the detector corresponding to the first half, the second Dopplerspread including a second Doppler shift, where the first and seconddetectors are divided by a first detector axis of division; determine afirst Doppler shift difference between the first and second Dopplershifts; determine a third Doppler spread for reflected light received ata third half of the detector, the third Doppler spread including a thirdDoppler shift; and determine a fourth Doppler spread for reflected lightreceived at a fourth half of the detector corresponding to the thirdhalf, the fourth Doppler spread including a fourth Doppler shift, wherethe third and fourth halves of the detector are separated along a seconddetector axis of division orthogonal to the first detector axis ofdivision; determine a second Doppler shift difference between the thirdand fourth Doppler shifts; and identify, a rotation axis of the targetaligned with a vector given by the sum of: a first vector aligned withthe first detector axis of division and having a magnitude given by thefirst Doppler shift difference; and a second vector aligned with thesecond detector axis of division and having a magnitude given by thesecond Doppler shift difference.

Embodiments of the computer readable medium may include one or more ofthe following.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors tocoherently add the Doppler spreads measured by at least two differentdetector segments forming each of said first, second, third, or fourthdetector half to determine the Doppler spread for reflected lightreceived at each of said first, second, third, and fourth detectorhalves, respectively.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors to:identify a width, Δf, of the Doppler spread of the reflected lightreceived over the entire field of view of the detector; and calculatethe magnitude of the target rotation according to: Δf=2DΩ/λ, where D isthe diameter of the target, Ω is the magnitude of the target rotation,and λ is the wavelength of the reflected light.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors to:coherently sum of the Doppler spreads measured for each detector segmentas the Doppler spread of the reflected light received over the entirefield of view of the detector; identify the peak of the Doppler spreadin the frequency domain; and identify the width of the Doppler spread tobe the full width of the Doppler spread at half the intensity from thepeak.

In an embodiment, a method of remotely measuring rotation of a target isprovided. The method includes: measuring, at receiver including adetector having a field of view divided into at least four spatiallydistinct light sensing segments, coherent light reflected from a targetincident upon the detector field of view, the incident coherent lightincluding a speckle pattern that moves over time with respect to thedetector field of view; outputting, by the detector, a plurality ofsignals to one or more processors, each signal representing an intensityof the reflected light received at a respective detector segment as afunction of time. The method further includes determining, by the one ormore processors: a first intensity as a function of time for reflectedlight received at a first half of the detector; a second intensity as afunction of time for reflected light received at a second half of thedetector corresponding to the second half, the first and second halvesof the detector separated along a first detector axis of division; athird intensity as a function of time for reflected light received at athird half of the detector; a fourth intensity as a function of time forreflected light received at a fourth half of the detector correspondingto the third half, the third and fourth halves of the detector separatedalong a second detector axis of division oriented orthogonal to thefirst detector axis of division. The method also includes measuring, bythe one or more processors, a first time delay representing a time forthe speckle pattern to move between the first and second detectorhalves; measuring, by the one or more processors, a second time delayrepresenting a time for the speckle pattern to move between the thirdand fourth detector halves; calculating, by the one or more processors,a linear velocity of the target, V, as the sum of: a first vectoraligned perpendicular to the first detector axis of division and havinga magnitude given by the measured first time delay; and a second vectoraligned perpendicular to the second detector axis of division and havinga magnitude given by the measured second time delay.

Embodiments of the method further include one or more of the following,in any combination.

In an embodiment, the method further includes, by the one or moreprocessors, calculating a rotational velocity of the target from thelinear target velocity, V.

In an embodiment of the method, the coherent light reflected from atarget is incident upon the detector field of view positioned at thepupil plane of the receiver.

In an embodiment of the method, measuring coherent light reflected fromthe target includes heterodyne detection of the coherent light reflectedfrom the target incident upon the detector field of view.

In an embodiment of the method, determining the light intensity as afunction of time for reflected light received at each of said first,second, third, and fourth detector halves includes, by the one or moreprocessors, coherently adding the signals measured by at least twodifferent detector segments forming said first, second, third, or fourthdetector half, respectively.

In an embodiment of the method, the first detector half includes a firstand a second detector segment; the second detector half includes a thirdand a fourth detector segment; the third detector half includes thefirst and the third detector segment; and the fourth detector halveincludes the second and the fourth detector segments.

In an embodiment of the method, measuring the first and second timedelays includes, by the one or more processors: calculating a firstcross-correlation as a function of time between the first intensity andthe second intensity; calculating a first cross-correlation as afunction of time between the third intensity and the fourth intensity;determining the first time delay as the time that maximizes the firstcross-correlation; and determining the second time delay as the timethat maximizes the second cross-correlation.

In an embodiment, a system for measuring rotation of a target isprovided. The system includes a detector including a field of viewdivided into at least four spatially distinct light sensing elements andone or more processors in communication with the detector. The detectoris adapted to: measure coherent light reflected from a target incidentupon the detector field of view, wherein the incident coherent lightincludes a speckle pattern that moves over time with respect to thedetector field of view; and output a plurality of signals, each signalrepresenting an intensity of the reflected light received at arespective detector segment as a function of time. The one or moreprocessors are adapted to: receive the plurality of signals; determine afirst intensity as a function of time for reflected light received at afirst half of the detector; determine a second intensity as a functionof time for reflected light received at a second half of the detectorcorresponding to the second half, the first and second halves of thedetector separated along a first detector axis of division; determine athird intensity as a function of time for reflected light received at athird half of the detector; determine a fourth intensity as a functionof time for reflected light received at a fourth half of the detectorcorresponding to the third half, the third and fourth halves of thedetector separated along a second detector axis of division orientedorthogonal to the first detector axis of division; measure a first timedelay representing a time for the speckle pattern to move between thefirst and second detector halves; measure a second time delayrepresenting a time for the speckle pattern to move between the thirdand fourth detector halves; calculate a linear velocity of the target,V, as, as the sum of: a first vector aligned perpendicular to the firstdetector axis of division and having a magnitude given by the measuredfirst time delay; and a second vector aligned perpendicular to thesecond detector axis of division and having a magnitude given by themeasured second time delay.

Embodiments of the system further include one or more of the following,in any combination.

In an embodiment, the system further includes a first light sourceadapted to emit a first coherent light beam having a first frequency; aplurality of optical focusing systems adapted to direct the firstcoherent light beam incident upon the target, and direct least a portionof the coherent light reflected from the target at reflected coherentlight upon the detector field of view; and a second light source adaptedto emit a second coherent light beam having a second frequency upon thedetector field of view, the second frequency different than the firstfrequency; where the plurality of signals output by the detector arebased upon interference of the reflected first coherent light beam andthe second coherent light beam.

In an embodiment of the system, the one or more processors are furtheradapted to coherently add the signals measured by at least two differentdetector segments forming each of said first, second, third, or fourthdetector half to determine the intensity as a function of time forreflected light received at each of said first, second, third, andfourth detector halves, respectively.

In an embodiment of the system, the first detector half includes a firstand a second detector segment; the second detector half includes a thirdand a fourth detector segment; the third detector half includes thefirst and the third detector segment; and the fourth detector halveincludes the second and the fourth detector segments.

In an embodiment of the system, the one or more processors are furtheradapted to calculate a rotational velocity of the target from the lineartarget velocity, V.

In an embodiment of the system, the detector field of view is positionedat the pupil plane of the receiver.

In an embodiment of the system, measuring the first and second timedelays includes, by the one or more processors: calculating a firstcross-correlation as a function of time between the first intensity andthe second intensity; calculating a first cross-correlation as afunction of time between the third intensity and the fourth intensity;determining the first time delay as the time that maximizes the firstcross-correlation; and determining the second time delay as the timethat maximizes the second cross-correlation.

In an embodiment, a non-transitory computer-readable medium havingcomputer-readable program codes embedded thereon is provided. Theprogram codes embedded thereon include instructions that, when executedby the one or more processors, cause the one or more processors to:receive a plurality of signals output from respective segments of adetector including at least four spatially distinct light sensingsegments, each signal representing an intensity of coherent lightreflected from a target and received at the respective detector segmentas a function of time, wherein the received light includes a specklepattern that moves over time with respect to the detector field of view;determine a first intensity as a function of time for reflected lightreceived at a first half of the detector; determine a second intensityas a function of time for reflected light received at a second half ofthe detector corresponding to the second half, the first and secondhalves of the detector separated along a first detector axis ofdivision; determine a third intensity as a function of time forreflected light received at a third half of the detector; determine afourth intensity as a function of time for reflected light received at afourth half of the detector corresponding to the third half, the thirdand fourth halves of the detector separated along a second detector axisof division oriented orthogonal to the first detector axis of division;measure a first time delay representing a time for the speckle patternto move between the first and second detector halves; measure a secondtime delay representing a time for the speckle pattern to move betweenthe third and fourth detector halves; calculate a linear velocity of thetarget, V, as, as the sum of: a first vector aligned perpendicular tothe first detector axis of division and having a magnitude given by themeasured first time delay; and a second vector aligned perpendicular tothe second detector axis of division and having a magnitude given by themeasured second time delay.

Embodiments of the computer-readable medium further include one or moreof the following, in any combination.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors tocoherently add the signals measured by at least two different detectorsegments forming each of said first, second, third, or fourth detectorhalf to determine the intensity of light reflected from the target andreceived at each of said first, second, third, and fourth detectorhalves, respectively, as a function of time.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors tocalculate a rotational velocity of the target from the linear targetvelocity, V.

In an embodiment, the computer-readable medium further includesinstructions that, when executed, cause the one or more processors to:calculate a first cross-correlation as a function of time between thefirst intensity and the second intensity; calculate a firstcross-correlation as a function of time between the third intensity andthe fourth intensity; determine the first time delay as the time thatmaximizes the first cross-correlation; and determine the second timedelay as the time that maximizes the second cross-correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will beapparent from the following more particular description of theembodiments, as illustrated in the accompanying drawings in which likereference characters refer to the same parts throughout the differentviews. The drawings are not necessarily to scale, emphasis instead beingplaced upon illustrating the principles of the embodiments.

FIGS. 1A-1B are schematic illustrations of an embodiment of a rotationmeasurement system for use in detecting a rotation magnitude anddirection of a target based upon laser light reflected from the target(“return signal”); (A) laser light incident upon a target; (B) laserlight reflected from the target (“return signal”);

FIG. 2A is a block diagram illustrating components of an embodiment ofthe rotation measurement system of FIG. 1, including a receiver housinga light detector in communication with an analyzer;

FIG. 2B is a schematic illustration of an embodiment of the receiver,demonstrating the positions of pupil and image planes at which thedetector may be positioned;

FIG. 3 is a block diagram illustrating embodiments of the detector andanalyzer of FIG. 2;

FIGS. 4A-4B illustrate simulated return signals measured according toembodiments of the disclosure; (A) single Doppler spread measured over acoherent integration time; (B) incoherent average of multiple Dopplerspreads, each measured over the coherent integration time;

FIGS. 5A-5B illustrate return signal measurement for corresponding firstand second halves of the detector of FIG. 3 separated by a firstdetector axis (e.g., an elevation axis) according to embodiments of thedisclosure; (A) return signal measurements of first and second detectorhalves individually; (B) Doppler spread difference between the returnsignal measurements of the first and second detector halves;

FIGS. 6A-6B illustrate return signal measurement for corresponding thirdand fourth halves of the detector of FIG. 3 separated by a seconddetector axis (e.g., an azimuth axis) according to embodiments of thedisclosure; (A) return signal measurements of third and fourth detectorhalves individually; (B) Doppler spread difference between the returnsignal measurements of the third and fourth detector halves;

FIGS. 7A-7B illustrate return signal measurement over the entire fieldof view of the detector of FIG. 3 according to embodiments of thedisclosure; (A) detector; (B) Doppler spread;

FIG. 8 is a block diagram illustrating an alternative embodiment of thedetector and analyzer of FIG. 2;

FIG. 9 illustrates a simulated return signal measured according toembodiments of the rotation measurement system of FIG. 8;

FIGS. 10A-10C illustrate return signal measurements over correspondingfirst and second halves of the detector according to embodiments of therotation measurement system of FIG. 8; (A) first and second detectorhalves separated along a first detector axis (B) intensity-time plotsfor the first and second detector halves; (C) cross-correlation of theintensity-time responses of FIG. 10B; and

FIGS. 11A-11C illustrate return signal measurements over correspondingthird and fourth halves of the detector according to embodiments of therotation measurement system of FIG. 8; (A) third and fourth detectorhalves separated along a second detector axis (B) intensity-time plotsfor the third and fourth detector halves; (C) cross-correlation of theintensity-time responses of FIG. 11B.

DETAILED DESCRIPTION

Embodiments of the disclosure are directed to systems and correspondingmethods for measuring rotation characteristics (e.g., rotation magnitudeand direction) of remote targets. The system includes a receiverhousing, including optics, that direct coherent light (e.g., laserlight) reflected from a target object onto the field of view of adetector. The detector measures the intensity and frequency of thereflected coherent light (referred to herein interchangeably as a returnsignal) and outputs a plurality of signals representative of themeasured reflected light to an analyzer. The analyzer employs themeasured intensity-time and frequency-time response of the reflectedlaser light to determine rotation characteristics of the target. Asdiscussed in greater detail below, in embodiments of a first detectiontechnique, the return signal is measured at the image plane of thereceiver optics, while in embodiments of a second detection technique,the return signal is measured at a pupil plane of the receiver optics.Measuring the return signal at these different locations allows theeffects of the target rotation on the return signal to be analyzed indifferent ways, providing alternative approaches to measurement oftarget rotation characteristics.

Use of laser light for determining rotation characteristics of a targetprovides a number of advantages over radar. For example, laser lightpropagates with much less divergence than radar, allowing laser light toreach targets at much larger ranges than radar. Furthermore, laser lightis much more sensitive to low rotation speeds than radar. For example, atarget object rotating at 1 mrad/sec would require about 10 minutes tobe resolved by radar but can be resolved in about 5 ms using laserlight. Additionally, laser light may be used to characterize targetrotations as low as about 0.1 mrad/sec.

In embodiments of the first detection technique, changes in thefrequency of return signals are measured by the detector at the imageplane of the receiver to determine the direction and magnitude of targetrotation. In general, light reflected from a rotating object willexperience a change in frequency, referred to as a Doppler shift. Thedirection and magnitude of the Doppler shift depend upon the rotationcharacteristics of the target at the point of reflection. For example,light reflected from portions of the target moving away from thedetector will decrease in frequency as compared to the incidentfrequency (i.e., a negative or red shift), while laser light reflectedfrom portions of the object moving towards the detector will increase infrequency (i.e., a positive or blue shift). The magnitude of the Dopplershift is proportional to the radial component of the target's surfacevelocity at the point of reflection, increasing from zero at therotation axis of the target to a maximum at the extreme edges of thetarget.

The return signal measured by the detector is representative of lightreflected from various locations on the target surface, each exhibitinga different Doppler shift of different magnitude. As a result, whenplotting the return signal in the form of intensity as a function offrequency, Doppler shifts are observed over a range of frequencies. Inthis context, the intensity distribution may be referred to as a Dopplerspread, while the frequency at the peak of the Doppler spread may bereferred to as its Doppler shift.

Embodiments of the detector are configured to take advantage of thespatial variation in the Doppler spread to detect the rotation magnitudeand direction of targets. The detector field of view is divided intomultiple segments, each capable of independently measuring the intensityand frequency of the return signal incident thereon. For example, thedetector segments may output electrical currents representative of theintensity and frequency of the reflected light. By analyzing the returnsignals measured at different detector segments, target rotation anddirection may be determined.

For example, assume a detector with four segments, arranged in equalsized quadrants. A first detector half (e.g., a top half) may be formedby first and second detector segments, while a corresponding seconddetector half (e.g., a bottom half) may be formed by third and fourthdetector segments. Likewise, a third detector half (e.g., a left half)may be formed by the first and third detector segments whilecorresponding fourth detector half (e.g., a right half) may be formed bythe second and fourth detector segments. So configured, the first andsecond halves are separated by a first detector axis of division, whilethe third and fourth halves are separated by a second detector axis ofdivision.

According to this first embodiment, the rotation magnitude of the targetmay be correlated with the width of the Doppler spread measured forreflected light incident upon the entire field of view of the detector.This Doppler spread may be calculated by combining the Doppler spreadsmeasured by all segments of the detector. Continuing the example above,the Doppler spread for the entire detector field of view is given by thecombination of the Doppler spreads measured at each of the four detectorsegments.

Embodiments of the first detection technique further determine thetarget rotation direction from differences in Doppler shifts measuredbetween corresponding halves of the detector. In general, the presenceof such a Doppler shift difference indicates that the return signalmeasured for one half of the detector is decreased or increased withrespect to the corresponding detector half. The physical implication ofthis observation is that at least a portion of the target rotation takesplace away from the detector half measuring a negative Doppler shift andtowards the corresponding half measuring a positive Doppler shift. Astarget's rotation axis marks the transition between rotation towards oraway from the detector, this observation can be alternatively understoodas indicating that at least a portion of the target's rotation takesplace about a virtual axis aligned with the detector axis of divisionseparating the corresponding detector halves. The orientation of thetarget's rotation axis can thought of as the sum of two orthogonalvectors, each having a magnitude and direction, where the direction ofeach vector is aligned with the detector axes, and the sign of eachvector is given according to a sign convention adopted for the analysis(e.g., a right handed rule).

To determine these two orthogonal vectors in the context of the detectorhaving four segments, assume that the top and bottom detector halves areeach formed from two segments and separated by a detector axis ofdivision aligned with the elevation axis. Further assume the left andright segments are likewise formed by two detector halves and separatedby a detector axis of division aligned with the azimuth axis. Taking thetwo orthogonal vectors to be aligned with the azimuth and elevationaxes, the magnitude of the Doppler shift difference between the top andbottom detector segments gives the magnitude of the elevation vector andthe magnitude of the Doppler shift difference between top and bottom ofthe detector gives the magnitude of the azimuth vector. Following aright handed rule convention, rotation towards the bottom and rightdetector halves is associated with the positive directions of theelevation and azimuth vectors, respectively, while rotation towards topand left detector halves is associated with negative directions of theelevation and azimuth vectors, respectively.

In embodiments of the second detection technique, changes in theintensity of return signals as a function of time are measured by thedetector at the pupil plane of the receiver, rather than the imageplane. In general, light is diffusely reflected from the target andexhibits random intensity changes as a function of time (e.g., a specklepattern). Due to the rotation of the target, when the speckle pattern isprojected onto a plane, it appears to move at a linear velocity that isrelated to the direction and magnitude of the target's rotation. Thus,by placing the detector in the pupil plane of the receiver, where thereceiver aperture is re-imaged on the detector field of view, the motionof the speckle pattern may be measured by the detector. Further analysisof the measured speckle pattern motion may be employed to determine itsvelocity, and thus the target's rotation.

For example, the detector employed in this second embodiment may be thesame as that discussed above with respect to the first embodiment,including a plurality of spatially distinct detector segments separatedby two orthogonal axes of division (e.g., aligned with the azimuth andelevation axes). The light intensity as a function of time for pairs ofcorresponding detector halves (e.g., top, bottom, left and right halves)is determined from the return signals of the respective detectorsegments. The target linear velocity is further partitioned into anazimuth vector and an elevation vector. The azimuth vector representsthe velocity of the speckle pattern normal to the azimuth axis (e.g.,motion between the left and right detector halves) and the elevationvector represents the velocity of the speckle pattern normal to theelevation axis (e.g., motion between the top and bottom detectors). Thetime required for the speckle pattern to move between orthogonal pairsof corresponding detector halves gives the magnitude and sign of thecomponent vectors of the target's linear velocity. With the azimuth andelevation vectors so determined, the linear and rotational velocity ofthe target may be determined.

As discussed in greater detail below, these times may be determined bycross-correlating the measured intensity as a function of time (e.g.,speckle pattern) for the detector halves. The cross-correlation providesa measure of the similarity of the speckle patterns at each detectorhalf for a given time delay between the two patterns and exhibits amaximum at the time delay for which the two speckle patterns are mostsimilar. Physically, this time delay represents the time required forthe speckle pattern to move from one detector half to its correspondingdetector half. Accordingly, the cross-correlation may be calculated inlight of the coordinate system adopted for the physical layout of therespective detector halves such that a negative time delay is providedfor movement of the speckle pattern in negative directions (e.g., fromright to left detector halves and top to bottom detector halves). Thus,in the second embodiment, the time delay of the cross-correlationmaximum between top and bottom detector halves may be employed as themagnitude of the elevation vector. Likewise, the time delay of thecross-correlation maximum between left and right detector halves may beemployed as the magnitude of the elevation vector.

The discussion will now turn to FIGS. 1A-1B, which present anillustrative environment 100 including a rotation measurement system 102illuminating a rotating target 104 with incident laser light 106A. Thetarget 104 possesses a diameter, D, perpendicular to the axis ofrotation, and rotates at angular velocity, Ω. The measurement system 102transmits laser light 106A incident upon the target 104 (FIG. 1A) andmeasures the laser light reflected from the target 106B (FIG. 1B). Suchmeasurements may include, but are not limited to, intensity andfrequency. The reflected laser light 106B may also be referred to hereinas return signal 106B.

The reflected laser light 106B will experience a Doppler shift due tothe rotation of the object, where the amount of the Doppler shift isproportional to the radial component of the surface velocity of therotating target 104. At the extreme edges of the target 104, the radialcomponent of the surface velocity, V, is given by Equation (1):

V=±DΩ/2  (1)

where +V is the radial component of the surface velocity at the edge ofthe target 104 rotating towards the system 102 and −V is the radialcomponent of the surface velocity at the edge of the target 104 rotatingaway from the system 102. In view of Equation 1, it may be furtherunderstood that locations nearer to the center of the target 104 willexhibit radial components of the surface velocity smaller than V,approaching zero at center of the target 104. Thus, the Doppler shiftexperienced by the reflected laser light 106 will depend upon thelocation of the target 104 from which it reflects.

A consequence of this variation in the Doppler shift is that thereflected light intensity measured by the rotation measurement system102 will possess different frequencies, spread over a range, Δf, givenaccording to Equation 2:

Δf=2/λ·2V=2/λ·DΩ  (2)

An embodiment of the rotation measurement system 102 is illustrated ingreater detail in FIG. 2. In an embodiment, the system 102 is capable ofperforming coherent or heterodyne detection of the coherent lightreflected from the target object 104. Optical frequencies oscillate toorapidly for direct electronic measurement and analysis of the electricfield of the return signal. Heterodyne detection is a process thatconverts the modulation of the reflected light into the same modulationon an electric current on a frequency less than that of the reflectedlight. Accordingly, heterodyne detection facilitates measurement of thereturn signal (e.g., relative phase, frequency, and/or amplitude of thereturn signal).

As illustrated in the embodiment of FIG. 2A, the system 102 includes alaser source 202, a transmit/receive switch 204, optics 206, a receiver210 housing a detector, a local oscillator (LO) 212, an analyzer 214,and a data storage device 216. Heterodyne detection utilizes thecoherent or single frequency nature of the incident laser light 106A(electromagnetic wave) generated by the laser source 202 to down convertthe modulation frequency of the reflected laser light 106B. In anembodiment, the laser source 202 may be adapted to emit laser light 106Ahaving a wavelength selected within the range between the ultravioletand far infrared regions of the electromagnetic spectrum (e.g., fromabout 100 nm to about 15 μm). In alternative embodiments, the wavelengthof the laser light 106A may be selected within the range of wavelengthsthat are understood to be safe for viewing by the human eye (e.g.,within the range between about 1 μm to about 2 μm). In anotherembodiment, the wavelength of the laser light 106A may be selectedwithin the range between about 1.5 μm to about 2 μm. In furtherembodiments, the laser source 202 may output laser light 106A at a powerwithin the range between about 1 W to about 10 kW. For example, thelaser source 202 may output laser light 106A at a power within the rangebetween about 1 W to about 500 W.

The incident and reflected laser lights 106A, 106B are directed by thetransmit/receive switch 204 and optics 206. In an embodiment, the switch204 is an optical transmit/receive switch adapted to separatetransmission of the incident light 106A from the laser source 202 to thetarget 104, reception of the reflected light 106B, and direction of thereflected light 106B to the receiver 210 housing the detector. In afurther embodiment, the optics 206 are configured to collimate theincident laser light 106A towards the target 104 and focus the reflectedlight 106B onto the receiver 210.

The reflected laser light 106B so directed is made to interfere with asecond coherent reference light 212A, generated by the local oscillator212. The result of this interference (e.g., laser interference fringes)is detected at the receiver 210. In order for the reflected andreference lights 106B, 212A to interfere, the reference light 212Aemitted by the LO 212 is both temporally coherent and spatiallycoherent. For example, the temporal coherence of the reference light212A may possess a coherence time greater than or equal to the timeinterval over which the interference between the reflected and referencelights 106B, 212A is measured by the detector 300 (located at receiver210). The spatial coherence of the reference light 212A may becharacterized by a coherence diameter greater than or equal to thediameter of the aperture of the receiver 210. As discussed herein,reference to detection of reflected laser light or return signal 106Bmay include detection of the interference of reflected light 106B andreference lights 212A.

As discussed in greater detail below, the detector 300 performsheterodyne detection of the return signal 106B and outputs a pluralityof electrical signals representing the intensity as a function offrequency of the reflected light 106B to the analyzer 214. With thisinformation, the analyzer 214 may determine the rotation magnitude androtation direction of the target object 104. The analyzer 214 may beincorporated within the receiver 210 or operate as a separate componentof the system 102. Once determined, the rotation magnitude and rotationdirection of the target object 104 may be further transmitted to datastorage device 216 for storage and subsequent retrieval. Alternativelyor additionally, the determined rotation magnitude and rotationdirection of the target object 104 may be transmitted to another systemfor further analysis (e.g., inverse synthetic aperture and tomography).

The data storage device 216 may include any storage device capable ofmaintaining the determined rotation magnitude and rotation direction ofthe target object 104. The data storage device 216 may further maintainadditional information for use by the analyzer 214 for calculation ofthe target rotation direction and/or rotation magnitude (e.g., Δf, λ, D,measured Doppler spreads, etc.). Examples include, but are not limitedto, magnetic and solid state storage devices. The data storage device216 may be in local communication with the system 102 or remotecommunication via a network.

Embodiments of the detection system 102 may be further configured withthe detector 300 at different positions within the receiver 210.Examples of different detector positions are illustrated with regards toFIG. 2B. For example, ignoring the switch and reference light for thesake of simplicity, consider the target 104 as positioned on one side ofreceiver optics 250 (e.g., a plurality of receiver lenses) at objectplane 252. The receiver optics 250 focus the reflected light 106Breceived at the receiver onto an image 104′ positioned at the image orfocal plane 254 of the receiver optics 250, passing through a receiveraperture 256. In one embodiment, detector 300, discussed below inregards to FIG. 3, may be positioned at the image plane 254. In analternative embodiment, detector 300′ may be positioned at a pupil plane260, where the receiver aperture is re-imaged, rather than at the imageplane 254. As discussed in greater detail below, the return signal 106Bmeasured at the image plane 254 and analyzed by the analyzer 214 asdiscussed with respect to FIGS. 3-7B or measured at the pupil plane 260and analyzed by the analyzer 214 as discussed with respect to FIGS.8-11C to determine the target rotation characteristics.

The discussion below will provide a brief description of the heterodynedetection process. For example, assume that the return signal 106B isgiven by A(t) sin(ωt+θ), where A(t) is the amplitude of theelectromagnetic field at time t, ω is the frequency of the signalcarrier frequency, and θ is the phase of the return signal 106B withrespect to the reference light 212A. Further assume that LO 212 is acontinuous laser emitting reference light 212A at an optical frequencyω′, slightly different than signal carrier frequency ω. The referencelight 212A may be expressed by B sin(ω′t). The sum of the two fields(e.g., 106B and 212A) at the face of the detector 300 (or detector 300′)is given by Equation 3:

Total Field=A(t)sin(ωt+θ)+B sin(ω′t)  (3)

The current (i) generated by the detector 300 (or detector 300′) uponincidence of the interference between the reflected and reference lights106A, 212A is proportional to the total field squared (Equations 4, 5):

i∝[A(t)sin(ωt+θ)+B sin(ω′t)]²  (4)

i∝A(t)² sin²(ωt+θ)+B ² sin²(ω′t)+2A(t)B sin(ωt+θ)sin(ω′t)=A(t)²sin²(ωt+θ)+B ² sin²(ω′t)+A(t)B[cos((ω−ω′)t+θ)−cos((ω+ω′)t+θ)]  (5)

Define the difference in optical carrier frequencies ω and ω′ asintermediate frequency ω_(if) (Equation 6):

ω_(if)=(ω−ω′)/2π  (6)

To simplify Equation 6, additional assumptions can be employed. Assumethat the detector intrinsic frequency response eliminates the doublefrequency components of Equation 5, cos(ω+ω′). Further assume that theintensity of the reference light 212A is much larger than that of thereflected light, such that B>>A. As a result, the term A(t)² sin²(ωt+θ)in Equation 5 is much less than B² sin²(ω′t) and can be ignored.Additionally assume that a high pass filter eliminates the DC term B²sin²(ω′t) in Equation 5 due to the reference light 212A. With theseassumptions, Equation 5 may be represented as Equation 5′:

i′A(t)B cos(ω_(if) t+θ)  (5′)

Two features are apparent from Equation 5′. First, the amplitude, B, ofthe field of the reference light 212A, appears as a gain multiplier ofthe amplitude, A, of the field of the reflected light, 106B. Second,provided that the LO carrier frequency, ω′, is well known, ω_(if)represents the frequency of the return signal 106B down shifted to anelectronically accessible sampling level as well as its phase, θ. Inother words, the changes in the frequency of the return signal 106B dueto a target reflection, such as the Doppler shift, can be measured usingcommon electronic components.

The discussion will now turn to FIG. 3, which illustrates embodiments ofthe detector 300 (positioned at the image plane 254 of the receiveroptics 250) and analyzer 214 in greater detail. The detector 300possesses a field of view divided into a plurality of spatially distinctlight sensing segments (i.e., a detector array). In certain embodiments,the detector 300 may possess at least three light sensing segments. Forthe purposes of the discussion, the detector 300 will be described withfour sensing segments, 300A, 300B, 300C, 300D, separated alongorthonormal detector axes of division 302, 304. So configured, thedetector segments are oriented as quadrants of the total detector fieldof view (e.g., a 2×2 array), as illustrated in FIG. 3.

As further discussed below, the return signals 106B measured by eachrespective detector segment may be combined. For example, with continuedreference to FIG. 3, combining the respective return signals 106B of anytwo segments will provide a return signal for half of the detector.Using the detector axes of division 302, 304 to define differentdetector halves, four different detector halves may be identified, eachcombining the return signals of two different pairs of detectorsegments. For example, a first detector half (e.g., a top detector half)is formed from the combination of detector segments 300A/300B. Similarlya second half (e.g., a bottom half), a third half (e.g., a left half),and a fourth half (e.g., a right half) may be formed from thecombination of detector segments 300C/300D, 300A/300C, and 300B/300D,respectively. As discussed herein, corresponding or complementarydetector halves may be detector halves separated by a detector axis ofdivision (e.g., top and bottom halves or left and right halves).Alternatively stated, the complementary detector halves do not overlap.

In alternative embodiments, the type of detector, the number of detectorsegments, and the relative orientation of the detector segments may bevaried. For example, in certain embodiments, the detector segments maybe linear detectors or Geiger mode avalanche photodiode detectors(GMAPDs). In further embodiments, the detector may include largernumbers of segments arranged in an array (e.g., a 4×4 array, a 5×5array, etc.). In such larger arrays, quadrants of the detector may bedefined by synthesizing the output of detector segments positionedwithin the respective physical quadrants of the detector. For example,in a 4×4 array, quadrants may be formed from 2×2 sub-arrays.

Each of the detector segments 300A-300D independently performsheterodyne detection of the return signal 106B received at itsrespective portion of the detector field of view and outputs, alongrespective channels (channels A-D), a plurality of signals (e.g.,current) representative of intensity and frequency of the return signal106B detected at each detector segment 300A-300D. For example, asdiscussed above, the plurality of signals output by the detector 300 mayinclude a plurality of currents having the same modulation as the returnsignal 106B with a carrier frequency ω_(if).

The discussion will now turn to FIGS. 4A-4B, which illustrate exemplarymeasurements of return signals 106B when the detector 300 is positionedat the image plane 254 of the receiver optics 250. Further discussion ofthe measurement of return signals 106B when the detector 300′ ispositioned at the pupil plane 260 of the receiver optics 250 is foundbelow in regards to FIG. 9.

A simulation of a return signal 106B including intensity (arbitraryunits) as a function of frequency is illustrated in FIG. 4A. Thesimulation assumes heterodyne detection of the return signal 106B over acoherent integration time (CIT) and a Doppler spread given by Equation2. The simulation further assumes Lambertian (diffuse) reflection of theincident laser light 106A, which is characteristic of targets 104 havinga relatively rough surface and causes the reflected laser light 106B tobe modulated with random amplitude changes, also referred to as speckle.As a result, the return signal of FIG. 4A is an intensity distribution,characterized by an intensity maximum or peak and a width extending overrange of frequencies to either side of the peak. The frequency of theintensity peak is referred to as the Doppler shift, while the intensitydistribution as a whole may be referred to as a Doppler spread. Specklenoise resulting from Lambertian reflection is also illustrated.

Notably, for a single measurement of the return signal 106B, such asFIG. 4A, the amplitude of the speckle noise is a significant fraction ofthe amplitude of the Doppler spread. However, the Doppler spread isapproximately constant in time, while the speckle noise is random.Accordingly, the return signal 106B may be heterodyne detected overmultiple coherent integration times and averaged, as illustrated in theembodiment of FIG. 4B, to improve the signal to noise ratio. Forexample, the average may be an incoherent average, where the magnitudeof each measured return signal 106B is squared and added together. Asshown in FIG. 4B, this averaging may significantly increase theamplitude of the Doppler spread as compared to that of the specklenoise. As a result, error in the target rotation magnitude and directioncalculated from measurements of the return signal 106B may be reduced.

In certain embodiments, the plurality of signals output by the detector300 may be provided to an analog to digital converter (e.g., A/Dconverter 306, FIG. 3) prior to receipt by the analyzer 214 (e.g.,Channels A, B, C, D). The A/D converter 306 may convert the outputcurrent (or corresponding voltage) to a representative digital number.The output of the A/D converter 306 may be received by the analyzer 214for determining the rotation magnitude and direction of the targetobject 104.

In an embodiment, the analyzer 214 may include one or more processorsadapted for analyzing the plurality of signals measured by the detector300. For the purpose of discussion, the analyzer 214 may be therepresented as components or modules capable of performing one or moreanalysis operations. However, it may be understood that such analysisoperations are performed in hardware, software, and/or combinationsthereof by the one or more processors.

The discussion will now turn to discussion of operations performed bythe analysis component 214 for determination of the target rotationdirection and magnitude when the detector 300 is positioned at the imageplane 254 in reference to the embodiments of FIGS. 5A-5B, 6A-6B, and7A-7B. Discussion of operations performed by the analysis component 214for determination of the target rotation direction and magnitude whenthe detector 300′ is positioned at the pupil plane 260 are in referenceto the embodiments of FIGS. 10A-10C and 11A-11C.

To perform the analysis, a rotation convention for the target rotationis chosen that defines a vector to represent the rotation direction. Forthe purpose of example, a right-hand rule will be employed in thediscussion below. However, it may be understood that embodiments of thedisclosure may be employed in combination with other rotationconventions, without limit. With a right-hand rule, the right hand isoriented such that fingers curl towards the palm in the direction oftarget rotation. The outstretched tip of the thumb of the right hand ispointed in the direction of the axis of rotation vector. Thetwo-dimensional projection of the axis of rotation vector on a plane maybe represented as the sum of two orthogonal vectors, each having amagnitude and a sign (i.e., positive or negative).

As discussed in detail below, measurement of the return signals 106B bythe detector 300 may be employed to determine this two-dimensionalprojection of the target rotation vector. For example, assume the firstvector is aligned with the first detector axis of division 302 and thesecond vector is aligned with the second detector axis of division 304.The component of the target rotation described by rotation about an axisaligned with the first detector axis of division 302 will be manifestedas a difference between the Doppler shift of the top and bottom halvesof the detector. Similarly, the component of target rotation describedby rotation about an axis aligned with the second detector axis ofdivision 304 will be manifested as a difference between the Dopplershift of the left and right halves of the detector 300. Thus, comparisonof the Doppler spreads for the corresponding halves of the detector 300defined by the detector axes of separation 302, 304 allows targetrotation to be spatially discriminated. Furthermore, as discussed ingreater detail below, measurement of the width total Doppler spread overthe entire field of view of the detector 300 characterizes the magnitudeof the target rotation.

The analyzer 214 may include an arithmetic conversion component 310, aspectral analysis component 312, and a differential Doppler analysiscomponent 314 for use in analyzing the return signals 106B received fromthe detector 300. For example, the arithmetic conversion component 310may be employed to combine the measured return signals 106B ofrespective detector segments and provide the Doppler spreads forrespective halves of the detector 300. In certain embodiments, theDoppler spreads for respective detector halves may be determined bycoherent addition of the Doppler spread of each detector segment forminga respective detector half.

The embodiments of FIGS. 5A and 6A illustrate division of the detector300 into pairs of corresponding halves along orthogonal axes. Forexample, with respect to FIG. 5A, first detector axis of division 302divides a first detector half, formed by segments 300A and 300B, from asecond detector half, formed by detector segments 300C and 300D. Theanalyzer 214 (e.g., using arithmetic conversion component 310) maydetermine first and second Doppler spreads for reflected light receivedat the first and second halves of the detector 300 by coherently addingthe Doppler spreads measured for the detector segments 300A and 300B andthe Doppler spreads measured for the detector segments 300C and 300D,respectively.

As further illustrated in FIG. 6A, a third detector half, formed bysegments 300A and 300C, is divided from a fourth detector half, formedby detector segments 300B and 300D, by the second detector axis ofdivision 304. The analyzer 214 (e.g., using arithmetic conversioncomponent 310) may determine third and fourth Doppler spreads forreflected light received at the third and fourth halves of the detector300 by coherently adding the Doppler spreads measured for the detectorsegments 300A and 300C and the Doppler spreads measured for the detectorsegments 300B and 300D, respectively.

In certain embodiments, the Doppler spreads calculated for the detectorsegment halves may be further normalized in order to account forvariations in signal sensitivity between the different detectorsegments. For example, the normalization may be the Doppler spread overthe total field of view of the detector 300 (e.g., the coherent sum300A+300B+300C+300D). Thus, the Doppler spread for the first detectorhalf may be given by (300A+300B)/(300A+300B+300C+300D), while theDoppler spread for the second detector half may be given by(300C+300D)/(300A+300B+300C+300D).

In certain embodiments, the first detector axis of division 302 may bereferred to an elevation axis and the second detector axis of division304 may be referred to as an azimuth axis. Furthermore, reference may bemade to the first and second detector segments forming an upper detectorhalf, the third and fourth detector segments forming a lower half of thedetector, the first and third detector segments forming a left detectorhalf, and the second and fourth detector segments forming a rightdetector half. It may be understood, however, that such orientations areprovided for illustrative purposes and that embodiments of the disclosedsystems and methods may be employed with any desired orientation.

In further embodiments, comparison of the Doppler shifts betweencorresponding detector halves may identify the presence of a portion ofthe target rotation occurring about an axis aligned with the detectoraxis of separation. In general, if a component of the target rotationoccurs about the detector axis of division between the respectivedetector halves, the Doppler shifts of the two Doppler spreads will beoffset.

For example with reference to FIGS. 5B and 6B, the Doppler spreads ofthe corresponding detector halves (e.g., top/bottom and left/right,respectively) are illustrated. It may be observed with respect to FIG.5B that the Doppler spread for the first half of the detector(300A+300B) exhibits a negative Doppler shift, as compared to theDoppler spread for the second half of the detector (300C+300D). Incontrast, with reference to FIG. 6B, the Doppler spread for the thirdhalf of the detector (300A+300C) is nearly identical to the Dopplerspread for the fourth half of the detector (300B+300D), with no Dopplershift difference observed.

The analyzer 214 (e.g., the differential Doppler analysis component 314)may detect the presence of target rotation by examining the Dopplerspread difference between corresponding detector halves. For example, asfurther illustrated in FIG. 5B, the Doppler spread difference betweenthe top and bottom halves exhibits a pronounced rise and fall, withopposing peaks joined by a non-zero slope crossing the line of zerointensity, referred to herein as a zero crossing. The analyzer 214(e.g., the differential Doppler analysis component 314) may detect thepresence of this differential Doppler zero crossing by detecting theopposing peaks with the zero crossing point lying there between. Incontrast, as illustrated in FIG. 6B, the Doppler spread differencebetween left and right halves exhibits no zero crossing. The analyzer214 (e.g., the differential Doppler analysis component 314) may detectthe absence of this differential Doppler zero crossing by detecting theabsence of opposing peaks with the zero crossing point lying therebetween.

The magnitude of the component vectors of the target rotation axisreflect the relative degree to which the target rotation axis is alignedwith each of the detector axes of division 302, 304. This magnitude maybe given by the Doppler shift difference of the Doppler spreads betweenthe corresponding detector halves. In the present example, the firstcomponent vector is aligned with the first detector axis of division 302and has a magnitude given by a first Doppler shift difference betweenthe first and second detector halves. Likewise, the second componentvector is aligned with the second detector axis of division 304 and hasa magnitude given by a second Doppler shift difference between the thirdand fourth detector halves.

The analyzer 214 may further analyze the Doppler spreads in order todeterminer the Doppler shift differences between corresponding detectorhalves and, therefore, the magnitudes of the component vectors of thetarget rotation axis. For example, the analyzer (e.g., the spectralanalysis component 312) may identify the peak in each of the Dopplerspreads of the first and second detector halves and calculate thefrequency difference (i.e., phase difference) between the two peaks. Asthe Doppler shift of each Doppler spread is the peak frequency, thisdifference directly measures the Doppler shift difference.

The sign of the component vectors of the target rotation axis aredetermined according to the assumed rotation convention. Following aright handed rule convention, rotation towards the bottom or rightdetector halves indicates a positive first or second vector,respectfully. Similarly, rotation towards the top or left detectorhalves indicates a negative first or second vector, respectfully. Theanalyzer 214 (e.g., spectral analysis component 312) may identify thedetector halves demonstrating a positive Doppler shift and apply theappropriate sign to the component vector. In the context of the presentexample, the rotation observed towards the bottom detector halfindicates that the first vector points in the negative direction, fromright to left. In the context of the example, as the second vector hasno magnitude (i.e., no rotation towards the right or left detector half)and, therefore, it is not necessary to determine its sign.

Having determined the magnitude and direction of the two vectorcomponents of the target rotation axis vector, the analyzer 214 (e.g.,the arithmetic conversion component 310) may identify the rotation axisof the target as the sum of two vector components. For example,continuing the current example, the sum of the first and second vectorsis the first vector, as the second rotation vector is zero. Thus,direction of the target rotation axis is given by the direction of thefirst rotation vector.

The discussion will now turn to calculation of the target rotationmagnitude, with reference to FIGS. 7A-7B. As discussed above withrespect to Equation 2, Δf=2DΩ/λ, the magnitude of the target rotation,Ω, is based upon the width of the Doppler spread of the return signal106B received over the field of view of the detector 300, Δf. Thus, withmeasurement of Δf, knowledge of the wavelength of the incident light106B, λ, and the target diameter, D, the analyzer 214 may calculate thetarget rotation magnitude, Ω. It may be understood that the wavelengthof the incident and reflected light may be assumed to be approximatelyequal

Assuming that the target 104 is barely resolved when the return signal106B is measured by the detector 300, D may be taken to be approximatelythe resolution of the optics 206. This value of D may be provided to theanalyzer 214 or retrieved from the data storage device 216.

The Doppler spread of the return signal 106B received over the field ofview of the detector 300 may be determined by combining the Dopplerspreads measured for each detector segment (FIG. 7A). For example, theanalyzer 214 (e.g., spectral analysis component 210) may calculate thetotal Doppler spread over the entire detector field of view bycoherently adding the Doppler spreads of the return signal 106B detectedby each of the detector segments (e.g., segments A-D).

With the total Doppler spread, the width of the total Doppler spread maybe characterized by the full width at half maximum (FIG. 7B). Theanalyzer 214 (e.g., spectral analysis component 312) may furthercalculate the width of the total Doppler spread, Δf, by identifying thepeak in the total Doppler spread and fitting the Doppler spread to afunction (e.g., a normal distribution). For example, the spectralanalysis component 312 may perform a Fourier analysis (e.g., a Fouriertransform, fast Fourier transformation (FFT), or discrete Fouriertransform (DFT) on the total Doppler spread and identify the peak of theDoppler spread (i.e., the Doppler shift) in the frequency domain. Thewidth of the Doppler spread may be defined by the full width of thefunction at half the intensity of the peak. Thus, with measurement of Δfand knowledge of D and λ, the target rotation magnitude, Ω, may becalculated according to Equation 2.

The discussion will now turn to embodiments of the second detectiontechnique, where the frequency spread observed in the return signal 106Breflected from the rotating target 104 is characterized by amplitudemodulation due motion of a speckle pattern across the plane of thedetector 300′.

As discussed above with regards to FIG. 2B, if the detector 300′ ispositioned at the pupil plane 260 of the receiver 210 (see, e.g.,detector 300′), where the receiver aperture 256 is re-imaged, thedetector 300′ “sees” a random intensity change as the light amplitudevaries due to the random intensity of the speckle pattern moving acrossthe detector field of view. By employing a detector 300′ having multipledetector segments for measurement of the return signal 106B, it ispossible to determine the direction of motion of the pattern.

For example, with reference to FIG. 8, embodiments of the detector 300′are illustrated in combination with an analyzer 214′ in greater detail.The detector 300′ possesses a field of view divided into a plurality ofspatially distinct light sensing segments (i.e., a detector array). Incertain embodiments, the detector 300′ may possess at least three lightsensing segments. For the purposes of the discussion, the detector 300′will be described with four sensing segments, separated alongorthonormal detector axes of division 302′, 304′. So configured, thedetector segments are oriented as quadrants of the total detector fieldof view (e.g., a 2×2 array) with elements A, B, C, and D as illustratedin FIG. 8. The speckle pattern 802 re-imaged at the pupil plane 260spans the receiver aperture 256, allowing each detector segment tomeasure the speckle pattern. Other than its position with respect to thereceiver optics 250, embodiments of the detector 300′ may be the same asthose discussed with respect to detector 300.

A simulation of a return signal 106B including intensity (arbitraryunits) as a function of frequency is illustrated in FIG. 9. Thesimulation assumes heterodyne detection of the return signal 106B over acoherent integration time (CIT), and a Doppler spread given by Equation2. The simulation further assumes Lambertian (diffuse) reflection of theincident laser light 106A, which is characteristic of targets 104 havinga relatively rough surface and causes the reflected laser light 106B tobe modulated with random amplitude changes (e.g., speckle). In certainembodiments, the plurality of signals output by the detector 300′ may beprovided to an analog to digital converter (A/D converter 306) prior toreceipt by the analyzer 214′ (e.g., Channels A, B, C, D), as discussedabove.

The discussion will now turn to operations performed by the analysiscomponent 214′ for determination of the target rotation direction andmagnitude when the detector 300′ is positioned at the pupil plane 260with further reference to FIG. 8 and the embodiments of FIGS. 10A-10Cand 11A-11C. In an embodiment, the analyzer 214′ may include one or moreprocessors adapted for analyzing the plurality of signals measured bythe detector 300′. For the purpose of discussion, the analyzer 214′ maybe the represented as components or modules capable of performing one ormore analysis operations. For example, as illustrated in FIG. 8, theanalyzer 214′ includes an arithmetic conversion component 310, across-correlation component 804, and a speckle analysis component 806for use in analyzing the return signals 106B received from the detector300′. However, it may be understood that analysis operations discussedin the context of such component may be performed in hardware, software,and/or combinations thereof by the one or more processors.

The motion of the speckle pattern across the detector field of view maybe partitioned into first and second orthogonal vectors. For example,the first vector is the component of speckle motion extending betweenthe first and second detector halves separated by the first detectoraxis of division 302′ and is aligned in a direction perpendicular tothis first axis 302′. Similarly, the second vector is the component ofspeckle motion extending between the third and fourth detector halvesseparated by the second detector axis of division 304′ and is aligned ina direction perpendicular to this second axis 304′.

The magnitude and sign of each vector may be further characterized bythe delay in time between detection of the speckle pattern incorresponding detector halves. The magnitude and sign of the firstvector is given by a first time delay between detection of the specklepattern in the corresponding detector halves separated by the firstdetector axis of division 302′ (e.g., the first and second detectorhalves). The magnitude and sign of the second vector is given by asecond time delay between detection of the speckle pattern in thecorresponding detector halves separated by the second detector axis ofdivision 304′ (e.g., the third and fourth detector halves).

In the discussion below, it will be assumed that the first and seconddetector axes of division 302′, 304′ are aligned with elevation andazimuth axes, respectively. With this orientation, the first vector isan elevation vector extending perpendicular to the elevation axis anddescribing the elevation component of the speckle motion. The magnitudeand direction of the elevation vector is given by a first time delaybetween detection of the speckle pattern in the top and bottom detectorhalves. Likewise, the second vector is an azimuth vector extendingperpendicular to the azimuth axis and describing the azimuth componentof the speckle motion. The magnitude and direction of the azimuth vectoris given by a second time delay between detection of the speckle patternin the left and right detector halves.

For example, embodiments of FIGS. 10A and 11A illustrate division of thedetector 300′ into pairs of corresponding halves along orthogonal axes.With respect to FIG. 10A, the first detector axis of division 302′(e.g., the elevation axis) divides the detector 300′ into a firstdetector half, formed by segments 300A and 300B (e.g., a top detectorhalf), from a second detector half, formed by detector segments 300C and300D (e.g., a bottom detector half). As further illustrated in FIG. 11A,the second detector axis of division 304′ (e.g., the azimuth axis)divides the detector 300′ into a third detector half, formed by segments300A and 300C (e.g., a left detector half), and a fourth detector half,formed by detector segments 300B and 300D (e.g., a right detector half).

The analyzer 214 (e.g., arithmetic conversion component 310) maydetermine the light intensity-time response for a given detector half bycoherently adding the intensity-time response of its respectivesegments. For example, continuing the example above, a first lightintensity-time response for reflected light received at the top half ofthe detector is given by the coherent sum of the intensity-time responsemeasured by the detector segments 300A, 300B. A second lightintensity-time response for reflected light received at thecorresponding bottom half of the detector is given by the coherent sumof the intensity-time response measured by the detector segments 300C,300D. Similarly, third and fourth light intensity-time responses forlight received at the left and right halves of the detector are given bythe coherent sums of the intensity-time response measured by detectorsegments 300A, 300C and 300B, 300D.

In certain embodiments, the light intensities calculated for thedetector halves may be further normalized in order to account forvariations in signal sensitivity between the different detectorsegments. For example, the normalization may be the light intensitymeasured over the total field of view of the detector 300′ as a functionof time (e.g., the coherent sum 300A+300B+300C+300D). Thus, the lightintensity as a function of time for the first detector half may be givenby (300A+300B)/(300A+300B+300C+300D), while the light intensify as afunction of time for the second detector half may be given by(300C+300D)/(300A+300B+300C+300D).

Simulated intensity-time responses for the corresponding halves of thedetector are illustrated in FIGS. 10B and 11B. FIG. 10B illustrates theintensity-time responses of the top detector half (300A+300B) and thebottom detector half (300C+300D), while FIG. 11B illustrates theintensity-time responses of the left detector half (300A+300C) and theright detector half (300B+300D). The simulations assume the limitingcase where the target axis of rotation is aligned with the elevationaxis of the detector. The target is further assumed to rotate towardsthe bottom half of the detector and away from top half of the detector.With this rotation of the target, the speckle pattern moves from thebottom detector half to the top detector half, without correspondingmovement from right to left or left to right detector halves.

For example, as illustrated in FIG. 10B, there is a time delay observedbetween the time-intensity response of the top detector half and thebottom half. This time delay represents the amount of time betweendetection of the speckle pattern at the bottom detector and thedetection of that same speckle pattern at the top detector half. That isto say, motion of the speckle pattern from the bottom detector half tothe top detector half.

With further reference to FIG. 11B, it may be observed that thetime-intensity of the left and right detector halves responses are notthe same at any time. This observation indicates that the specklepattern detected at the left detector half is not the same patterndetected at the right detector half at any time. That is to say, thespeckle pattern does not move from the left to right detector half orthe right to left detector half.

As discussed above, the cross-correlation between top and bottomdetector halves is performed to obtain the first time delaycharacterizing the magnitude and sign of the elevation vector.Similarly, the cross-correlation between the left and right detectorhalves is performed to obtain the time delay characterizing themagnitude and sign of the azimuth vector. In an embodiment, the analyzer214′ (e.g., cross-correlation component 804) performs thecross-correlation according to Equation 7:

$\begin{matrix}{{{CORR}\left( {X,Y} \right)} = \frac{\sum\limits_{i = 1}^{n}{\left( {X_{i} - \overset{\_}{X}} \right)\left( {Y_{i} - \overset{\_}{Y}} \right)}}{\sqrt{\sum\limits_{t = 1}^{n}\left( {X_{t} - \overset{\_}{X}} \right)^{2}}\sqrt{\sum\limits_{t = 1}^{n}\left( {Y_{t} - \overset{\_}{Y}} \right)^{2}}}} & (7)\end{matrix}$

where X and Y are the intensity-time responses calculated forcorresponding detector halves, i is an index on time ranging from 1 ton, X_(i) and Y_(i) are the reflected light intensity sampled at twocorresponding detector halves for time index i, and X and Y are themeans of X_(i) and Y_(i), respectively.

The cross-correlation of Equation 7 characterizes the degree to which Xand Y are alike as a function of the time delay applied between the tworesponses. This operation can be thought of as holding theintensity-time response of one detector half fixed and shifting theintensity-time response of the corresponding detector half along thetime axis. The cross-correlation exhibits a maximum at the time delayfor which the intensity-time responses of the two detector halvesoverlap (i.e., are most alike). The analyzer 214′ (e.g., the speckleanalysis component 806, may determine the time delay at the peakaccording to techniques understood in the art for identifying theargument maximizing a function.

The selection of which of the corresponding detector halves is X andwhich is Y in Equation 7 may be made such that the time delay isnegative for speckle motion in the negative direction of the coordinatesystem assumed for the analysis. Thus, the time delay of thecross-correlation maximum quantifies the magnitude and sign of thevector components of the speckle velocity.

Continuing the example above, FIG. 10C presents the cross-correlation ofintensity-time response of the top detector half (300A+300B) and thebottom detector half (300C+300D), notated as CORR(300A+300B, 300C+300D).FIG. 11C illustrates an embodiment of the cross-correlation between theintensity-time response of the left detector half (300A+300C) and theright detector half (300B+300D), notated as CORR(300A+300C, 300B+300D).Each cross-correlation plots a cross-correlation parameter in arbitraryunits along the vertical axis and the time delay along the horizontalaxis. With reference to FIG. 10C, a peak (i.e., maximum) is observed atthe time delay where the time-intensity responses of the top and bottomdetector halves are most alike. In contrast, the cross-correlation ofFIG. 11C does exhibit a peak at any time delay. These results areconsistent with the observations discussed above with respect to FIGS.10B and 10C.

As discussed above, the linear velocity, V, of the speckle motion isgiven by the sum of the first and second vectors. The target's angularrotation vector, Ω, is calculated from this linear velocity according toEquation 8:

Ω=V/Range  (8)

where Range is the distance between the system 102 and the target 104.In certain embodiments, the analyzer 214′ may obtain the range from thedata storage device 216 for use in determining the target's angularrotation.

The above-described systems and methods can be implemented in digitalelectronic circuitry, in computer hardware, firmware, and/or software.The implementation can be as a computer program product. Theimplementation can, for example, be in a machine-readable storagedevice, for execution by, or to control the operation of, dataprocessing apparatus. The implementation can, for example, be aprogrammable processor, a computer, and/or multiple computers.

A computer program can be written in any form of programming language,including compiled and/or interpreted languages, and the computerprogram can be deployed in any form, including as a stand-alone programor as a subroutine, element, and/or other unit suitable for use in acomputing environment. A computer program can be deployed to be executedon one computer or on multiple computers at one site.

Method steps can be performed by one or more programmable processorsexecuting a computer program to perform functions of the invention byoperating on input data and generating output. Method steps can also beperformed by and an apparatus can be implemented as special purposelogic circuitry. The circuitry can, for example, be a FPGA (fieldprogrammable gate array) and/or an ASIC (application-specific integratedcircuit). Subroutines and software agents can refer to portions of thecomputer program, the processor, the special circuitry, software, and/orhardware that implement that functionality.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor receives instructions and data from a read-only memory or arandom access memory or both. The essential elements of a computer are aprocessor for executing instructions and one or more memory devices forstoring instructions and data. Generally, a computer can include, can beoperatively coupled to receive data from and/or transfer data to one ormore mass storage devices for storing data (e.g., magnetic,magneto-optical disks, or optical disks).

Data transmission and instructions can also occur over a communicationsnetwork. Information carriers suitable for embodying computer programinstructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices. Theinformation carriers can, for example, be EPROM, EEPROM, flash memorydevices, magnetic disks, internal hard disks, removable disks,magneto-optical disks, CD-ROM, and/or DVD-ROM disks. The processor andthe memory can be supplemented by, and/or incorporated in specialpurpose logic circuitry.

To provide for interaction with a user, the above described techniquescan be implemented on a computer having a display device. The displaydevice can, for example, be a cathode ray tube (CRT) and/or a liquidcrystal display (LCD) monitor. The interaction with a user can, forexample, be a display of information to the user and a keyboard and apointing device (e.g., a mouse or a trackball) by which the user canprovide input to the computer (e.g., interact with a user interfaceelement). Other kinds of devices can be used to provide for interactionwith a user. Other devices can, for example, be feedback provided to theuser in any form of sensory feedback (e.g., visual feedback, auditoryfeedback, or tactile feedback). Input from the user can, for example, bereceived in any form, including acoustic, speech, and/or tactile input.

The above described techniques can be implemented in a distributedcomputing system that includes a back-end component. The back-endcomponent can, for example, be a data server, a middleware component,and/or an application server. The above described techniques can beimplemented in a distributing computing system that includes a front-endcomponent. The front-end component can, for example, be a clientcomputer having a graphical user interface, a Web browser through whicha user can interact with an example implementation, and/or othergraphical user interfaces for a transmitting device. The components ofthe system can be interconnected by any form or medium of digital datacommunication (e.g., a communication network). Examples of communicationnetworks include a local area network (LAN), a wide area network (WAN),the Internet, wired networks, and/or wireless networks.

The system can include clients and servers. A client and a server aregenerally remote from each other and typically interact through acommunication network. The relationship of client and server arises byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

Packet-based networks can include, for example, the Internet, a carrierinternet protocol (IP) network (e.g., local area network (LAN), widearea network (WAN), campus area network (CAN), metropolitan area network(MAN), home area network (HAN)), a private IP network, an IP privatebranch exchange (IPBX), a wireless network (e.g., radio access network(RAN), 802.11 network, 802.16 network, general packet radio service(GPRS) network, HiperLAN), and/or other packet-based networks.Circuit-based networks can include, for example, the public switchedtelephone network (PSTN), a private branch exchange (PBX), a wirelessnetwork (e.g., RAN, bluetooth, code-division multiple access (CDMA)network, time division multiple access (TDMA) network, global system formobile communications (GSM) network), and/or other circuit-basednetworks.

The transmitting device can include, for example, a computer, a computerwith a browser device, a telephone, an IP phone, a mobile device (e.g.,cellular phone, personal digital assistant (PDA) device, laptopcomputer, electronic mail device), and/or other communication devices.The browser device includes, for example, a computer (e.g., desktopcomputer, laptop computer) with a world wide web browser (e.g.,Microsoft® Internet Explorer® available from Microsoft Corporation,Mozilla® Firefox available from Mozilla Corporation). The mobilecomputing device includes, for example, a Blackberry®.

The terms comprise, include, and/or plural forms of each are open endedand include the listed parts and can include additional parts that arenot listed. The term and/or is open ended and includes one or more ofthe listed parts and combinations of the listed parts.

One skilled in the art will realize the invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting of theinvention described herein. Scope of the invention is thus indicated bythe appended claims, rather than by the foregoing description, and allchanges that come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed is:
 1. A method of remotely measuring rotation of atarget, comprising: measuring, at receiver including a detector having afield of view divided into at least four spatially distinct lightsensing segments, coherent light reflected from a target incident uponthe detector field of view, the incident coherent light including aspeckle pattern that moves over time with respect to the detector fieldof view; outputting, by the detector, a plurality of signals to one ormore processors, each signal representing an intensity of the reflectedlight received at a respective detector segment as a function of time;and determining, by the one or more processors: a first intensity as afunction of time for reflected light received at a first half of thedetector; a second intensity as a function of time for reflected lightreceived at a second half of the detector corresponding to the secondhalf, the first and second halves of the detector separated along afirst detector axis of division; a third intensity as a function of timefor reflected light received at a third half of the detector; a fourthintensity as a function of time for reflected light received at a fourthhalf of the detector corresponding to the third half, the third andfourth halves of the detector separated along a second detector axis ofdivision oriented orthogonal to the first detector axis of division;measuring, by the one or more processors, a first time delayrepresenting a time for the speckle pattern to move between the firstand second detector halves; measuring, by the one or more processors, asecond time delay representing a time for the speckle pattern to movebetween the third and fourth detector halves; calculating, by the one ormore processors, a linear velocity of the target, V, as the sum of: afirst vector aligned perpendicular to the first detector axis ofdivision and having a magnitude given by the measured first time delay;and a second vector aligned perpendicular to the second detector axis ofdivision and having a magnitude given by the measured second time delay.2. The method of claim 1, further comprising, by the one or moreprocessors, calculating a rotational velocity of the target from thelinear target velocity, V.
 3. The method of claim 1, wherein thecoherent light reflected from a target is incident upon the detectorfield of view positioned at the pupil plane of the receiver.
 4. Themethod of claim 1, wherein measuring coherent light reflected from thetarget comprises heterodyne detection of the coherent light reflectedfrom the target incident upon the detector field of view.
 5. The methodof claim 1, wherein determining the light intensity as a function oftime for reflected light received at each of said first, second, third,and fourth detector halves comprises, by the one or more processors,coherently adding the signals measured by at least two differentdetector segments forming said first, second, third, or fourth detectorhalf, respectively.
 6. The method of claim 5, wherein: the firstdetector half comprises a first and a second detector segment; thesecond detector half comprises a third and a fourth detector segment;the third detector half comprises the first and the third detectorsegment; and the fourth detector halve comprise the second and thefourth detector segments.
 7. The method of claim 1, wherein measuringthe first and second time delays comprises, by the one or moreprocessors: calculating a first cross-correlation as a function of timebetween the first intensity and the second intensity; calculating afirst cross-correlation as a function of time between the thirdintensity and the fourth intensity; determining the first time delay asthe time that maximizes the first cross-correlation; and determining thesecond time delay as the time that maximizes the secondcross-correlation.
 8. A detection system for measuring rotation of atarget, comprising: a detector including a field of view divided into atleast four spatially distinct light sensing elements, the detectoradapted to: measure coherent light reflected from a target incident uponthe detector field of view, wherein the incident coherent light includesa speckle pattern that moves over time with respect to the detectorfield of view; and output a plurality of signals, each signalrepresenting an intensity of the reflected light received at arespective detector segment as a function of time; one or moreprocessors in communication with the detector, the one or moreprocessors adapted to: receive the plurality of signals; determine afirst intensity as a function of time for reflected light received at afirst half of the detector; determine a second intensity as a functionof time for reflected light received at a second half of the detectorcorresponding to the second half, the first and second halves of thedetector separated along a first detector axis of division; determine athird intensity as a function of time for reflected light received at athird half of the detector; determine a fourth intensity as a functionof time for reflected light received at a fourth half of the detectorcorresponding to the third half, the third and fourth halves of thedetector separated along a second detector axis of division orientedorthogonal to the first detector axis of division; measure a first timedelay representing a time for the speckle pattern to move between thefirst and second detector halves; measure a second time delayrepresenting a time for the speckle pattern to move between the thirdand fourth detector halves; calculate a linear velocity of the target,V, as, as the sum of: a first vector aligned perpendicular to the firstdetector axis of division and having a magnitude given by the measuredfirst time delay; and a second vector aligned perpendicular to thesecond detector axis of division and having a magnitude given by themeasured second time delay.
 9. The detector system of claim 8, furthercomprising: a first light source adapted to emit a first coherent lightbeam having a first frequency; a plurality of optical focusing systemsadapted to direct the first coherent light beam incident upon thetarget, and direct least a portion of the coherent light reflected fromthe target at reflected coherent light upon the detector field of view;and a second light source adapted to emit a second coherent light beamhaving a second frequency upon the detector field of view, the secondfrequency different than the first frequency; wherein the plurality ofsignals output by the detector are based upon interference of thereflected first coherent light beam and the second coherent light beam.10. The detector system of claim 8, wherein the one or more processorsare further adapted to coherently add the signals measured by at leasttwo different detector segments forming each of said first, second,third, or fourth detector half to determine the intensity as a functionof time for reflected light received at each of said first, second,third, and fourth detector halves, respectively.
 11. The detectionsystem of claim 10, wherein: the first detector half comprises a firstand a second detector segment; the second detector half comprises athird and a fourth detector segment; the third detector half comprisesthe first and the third detector segment; and the fourth detector halvecomprise the second and the fourth detector segments.
 12. The detectorsystem of claim 8, wherein the one or more processors are furtheradapted to calculate a rotational velocity of the target from the lineartarget velocity, V.
 13. The detector system of claim 8, wherein thedetector field of view is positioned at the pupil plane of the receiver.14. The detector system of claim 8, wherein measuring the first andsecond time delays comprises, by the one or more processors: calculatinga first cross-correlation as a function of time between the firstintensity and the second intensity; calculating a firstcross-correlation as a function of time between the third intensity andthe fourth intensity; determining the first time delay as the time thatmaximizes the first cross-correlation; and determining the second timedelay as the time that maximizes the second cross-correlation.
 15. Anon-transitory computer-readable medium having computer-readable programcodes embedded thereon including instructions that, when executed by theone or more processors, cause the one or more processors to: receive aplurality of signals output from respective segments of a detectorincluding at least four spatially distinct light sensing segments, eachsignal representing an intensity of coherent light reflected from atarget and received at the respective detector segment as a function oftime, wherein the received light includes a speckle pattern that movesover time with respect to the detector field of view; determine a firstintensity as a function of time for reflected light received at a firsthalf of the detector; determine a second intensity as a function of timefor reflected light received at a second half of the detectorcorresponding to the second half, the first and second halves of thedetector separated along a first detector axis of division; determine athird intensity as a function of time for reflected light received at athird half of the detector; determine a fourth intensity as a functionof time for reflected light received at a fourth half of the detectorcorresponding to the third half, the third and fourth halves of thedetector separated along a second detector axis of division orientedorthogonal to the first detector axis of division; measure a first timedelay representing a time for the speckle pattern to move between thefirst and second detector halves; measure a second time delayrepresenting a time for the speckle pattern to move between the thirdand fourth detector halves; calculate a linear velocity of the target,V, as, as the sum of: a first vector aligned perpendicular to the firstdetector axis of division and having a magnitude given by the measuredfirst time delay; and a second vector aligned perpendicular to thesecond detector axis of division and having a magnitude given by themeasured second time delay.
 16. The computer-readable medium of claim15, further including instructions that, when executed, cause the one ormore processors to coherently add the signals measured by at least twodifferent detector segments forming each of said first, second, third,or fourth detector half to determine the intensity of light reflectedfrom the target and received at each of said first, second, third, andfourth detector halves, respectively, as a function of time.
 17. Thecomputer-readable medium of claim 15, further including instructionsthat, when executed, cause the one or more processors to calculate arotational velocity of the target from the linear target velocity, V.18. The computer-readable medium of claim 15, further includinginstructions that, when executed, cause the one or more processors to:calculate a first cross-correlation as a function of time between thefirst intensity and the second intensity; calculate a firstcross-correlation as a function of time between the third intensity andthe fourth intensity; determine the first time delay as the time thatmaximizes the first cross-correlation; and determine the second timedelay as the time that maximizes the second cross-correlation.